Metre

=== Timeline ===
{|class="wikitable"
!width=130pt|Date!!Deciding body!!Decision
|-
| 8 May 1790||[[National Assembly (French Revolution)|French National Assembly]]||The length of the new metre to be equal to the length of a [[pendulum]] with a half-[[period (physics)|period]] of one [[second]].<ref name="Larousse">{{Cite book |last=Larousse |first=Pierre |url=https://gallica.bnf.fr/ark:/12148/bpt6k205363w |title=Grand dictionnaire universel du XIXe siècle : français, historique, géographique, mythologique, bibliographique.... T. 11 MEMO-O / par M. Pierre Larousse |date=1866–1877 |pages=163 |language=EN}}</ref>
|-
|30 Mar 1791||French National Assembly||Accepts the proposal by the [[French Academy of Sciences]] that the new definition for the metre be equal to one ten-millionth of the length of a great circle [[Circular sector|quadrant]] along the Earth's [[meridian (geography)|meridian]] through Paris, that is the distance from the equator to the north pole along that quadrant.{{sfn|Bigourdan1901|pp=20–21}}
|-
|1795||colspan=2|Provisional metre bar made of brass and based on [[Paris meridian|Paris meridan]] arc (French: ''Méridienne de France'') measured by [[Nicolas-Louis de Lacaille|Nicolas-Louis de Lacaillle]] and [[César-François Cassini de Thury|Cesar-François Cassini de Thury]], legally equal to 443.44 [[Line (unit)|lines]] of the ''toise du Pérou'' (a standard [[Units of measurement in France before the French Revolution#Length|French unit of length]] from 1766).<ref name="Larousse" /><ref name="Levallois" /><ref name="Wolf">{{Cite book |last=Wolf |first=Charles (1827–1918) Auteur du texte |url=https://gallica.bnf.fr/ark:/12148/bpt6k9807509c |title=Recherches historiques sur les étalons de poids et mesures de l'Observatoire et les appareils qui ont servi à les construire / par M. C. Wolf... |date=1882 |pages=C.38–39, C.2–4 |language=FR}}</ref><ref name="entreprises">{{Cite web |title=Histoire du mètre |url=https://www.entreprises.gouv.fr/metrologie/histoire-metre |access-date=2019-05-16 |website=Direction Générale des Entreprises (DGE) |language=fr}}</ref>  [The line was 1/864 of a ''toise''.]
|-
|10 Dec 1799||French National Assembly||Specifies the platinum metre bar, presented on 22 June 1799 and deposited in the [[National Archives of France|National Archives]], as the final standard. Legally equal to 443.296 lines on the ''toise du Pérou''.<ref name="entreprises" />
|-
|24–28 Sept 1889||1st [[General Conference on Weights and Measures]] (CGPM)||Defines the metre as the distance between two lines on a standard bar of an alloy of [[platinum]] with 10% [[iridium]], measured at the melting point of ice.<ref name="entreprises" /><ref>{{Cite web|url=https://www.bipm.org/utils/common/pdf/CGPM/CGPM1.pdf|title=CGPM : Compte rendus de la 1ère réunion (1889).|website=BIPM}}</ref>
|-
|27 Sept – 6 Oct 1927||7th CGPM||Redefines the metre as the distance, at {{convert|0|C|K|abbr=on|lk=on}}, between the axes of the two central lines marked on the prototype bar of platinum–iridium, this bar being subject to one standard [[atmospheric pressure|atmosphere of pressure]] and supported on two cylinders of at least 10&nbsp;mm (1&nbsp;cm) diameter, symmetrically placed in the same horizontal plane at a distance of 571&nbsp;mm (57.1&nbsp;cm) from each other.<ref>{{Cite web|url=https://www.bipm.org/utils/common/pdf/CGPM/CGPM7.pdf|title=CGPM : Comptes rendus de le 7e réunion (1927)|page=49}}</ref>
|-
|14 Oct 1960||11th CGPM||Defines the metre as {{val|fmt=spaces| 1650763.73}} [[wavelength]]s in [[vacuum]] of the [[electromagnetic radiation|radiation]] corresponding to the transition between the 2p{{sup|10}} and 5d{{sup|5}} quantum levels of the [[krypton]]-86 [[atom]].{{sfn|Judson|1976}}
|-
|21 Oct 1983||17th CGPM||Defines the metre as the length of the path travelled by [[light]] in vacuum during a time interval of {{sfrac|299 792 458}} of a [[second]].<ref>[[#taylor2008a|Taylor and Thompson (2008a), Appendix 1, p. 70.]]</ref><ref>{{cite web |url=https://www.nationalgeographic.org/thisday/oct21/meter-redefined/ |title=Meter is Redefined |publisher=National Geographic Society |location=US |access-date=2019-10-22 }}</ref>
|-
|2002||[[International Committee for Weights and Measures]] (CIPM)||Considers the metre to be a unit of [[proper length]] and thus recommends this definition be restricted to "lengths ℓ which are sufficiently short for the effects predicted by [[general relativity]] to be negligible with respect to the uncertainties of realisation".<ref name="taylor2008a77">[[#taylor2008a|Taylor and Thompson (2008a), Appendix 1, p. 77.]]</ref>
|}

{| class="wikitable" style="margin:0 auto;"
|+Definitions of the metre since 1795{{sfn|Cardarelli|2003}}
! Basis of definition
! Date
! Absolute<br>uncertainty
! Relative<br>uncertainty
|-
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| {{sfrac|10 000 000}} part of the [[Circular sector|quadrant]] along the [[meridian (geography)|meridian]], astronomical measure by [[Friedrich Bessel|Bessel]] (443.44 lines)
| 1792
| {{val|0.5|–|0.1|u=mm}}
| {{val|e=-4}}
|-
-->
| {{sfrac|{{val|10000000}}}} part of the [[Circular sector|quadrant]] along the [[meridian (geography)|meridian]], measurement by [[Jean Baptiste Joseph Delambre|Delambre]] and [[Pierre Méchain|Méchain]] (443.296 lines)
| 1795
| {{val|500|–|100|u=um}}
| {{val|e=-4}}
|-
| First prototype ''{{lang|fr|[[History of the metre#Mètre des Archives|Mètre des Archives]]}}'' platinum bar standard
| 1799
| {{val|50|–|10|u=um}}
| {{val|e=-5}}
|-
| Platinum–iridium bar at melting point of ice (1st [[CGPM]])
| 1889
| {{val|0.2|–|0.1|u=um}}
| {{val|e=-7}}
|-
| Platinum–iridium bar at melting point of ice, atmospheric pressure, supported by two rollers (7th CGPM)
| 1927
| n.a.
| n.a.
|-
| [[Hyperfine structure|Hyperfine]] atomic transition; {{val|1650763.73}} wavelengths of light from a specified transition in [[krypton-86]] (11th CGPM)
| 1960
| {{val|4|u=nm}}
| {{val|4e-9}}<ref>[http://www.bipm.org/en/CGPM/db/17/1/ Definition of the metre] Resolution 1 of the 17th meeting of the CGPM (1983)</ref>
|-
| Length of the path travelled by light in vacuum in {{sfrac|{{val|299792458}}}} second (17th CGPM)
| 1983
| {{val|0.1|u=nm}}
| {{val|e=-10}}
|}